1. Field of the Invention
The present invention relates to a bearing device for rotatably supporting a shaft member by a dynamic pressure action of fluid, which is generated in a bearing gap. This type of the bearing device is called a “fluid dynamic bearing device”, and is suitable for use in a spindle motor mounted in an information apparatus, for example, a magnetic disk device such as an HDD and an FDD, an optical disk device such as a CD-ROM/RAM, or a magneto-optical disk device such as an MD/MO, in a polygon scanner motor mounted in a laser beam printer (LBP) or the like, or in a small motor mounted in an electrical apparatus such as an axial fan.
2. Description of the Related Art
In the fluid dynamic bearing device, usually, a shaft member is supported in a non-contact fashion with a bearing member in a radial direction by a dynamic pressure action of fluid, which is generated in a radial bearing gap. In order to generate the dynamic pressure action of the fluid in the radial bearing gap, dynamic pressure grooves arranged, for example, in a herringbone configuration are formed as a dynamic pressure generating portion in a predetermined region of at least any one of an outer circumferential surface of the shaft member and an inner circumferential surface of the bearing member, which is opposite thereto.
Meanwhile, for the above-mentioned fluid dynamic bearing device for the information apparatus, extremely high rotational accuracy is required, and hence, it is necessary to form the dynamic pressure grooves with extremely high accuracy. As a method of forming this type of the dynamic pressure grooves on the inner circumferential surface of the bearing member, a form rolling is known (for example, see JP 10-196640 A).
In the invention described in JP 10-196640 A described above, a rolling machine having a plurality of protrusions is inserted into the inner circumference of the bearing member, and the rolling machine is then allowed to travel thereinto in an axial direction while being rotated alternately in a clockwise direction and a counter clockwise direction, thereby forming the dynamic pressure grooves in the inner circumference of the bearing member. In the method of forming the dynamic pressure grooves by the form rolling as described above, variation is prone to occur in a shape of the grooves owing to characteristics of the method, and it is difficult to mass-produce highly accurate grooves stably at low cost.
Further, for the above-mentioned bearing device for use in the information apparatus, much higher rotational accuracy is required. Therefore, it becomes important to set, with high accuracy, a gap (clearance) between the inner circumferential surface of the bearing member and the outer circumferential surface of the shaft member, which is opposite thereto.